October 16, 2008

Today's project was a broken 259. The modulation oscillator was not putting out anything in the sawtooth position of the switch. The Harmonics Order control was doing nothing, but voltage was effecting it. Judging simply from the outside of this module, I can assume it was not built at the Buchla factory, but was a kit purchased at the garage sale a few years ago. The blue LEDs (yuck!) and the black big knobs were the first tip off.

Once I had this guy open, I was treated to confusing extra wires, which I came to see were there to get around using the remote switch as intended and to use it to engage some extra trim pots. I guess that the switch puts the trimmed CV ins into 1.2v/oct mode to work with a 200e system. Why the builder didn't just add some fixed resistors and make the CV ins ALWAYS 1.2v/oct is beyond me.

The Harmonics Order control problem was just some bad soldering. During assembly the iron was held on one of the pot's pads for too long and it lifted off the board. Finally, the pad tore away from the trace.

The modulation oscillator turned out to be putting out a sawtooth wave, but it wasn't getting to the output. The LF13331 is the switching chip that I always HOPE isn't blown, since it is hard to come by. In this case, the working saw wave told me that the chip was fine. It turned out to be a faulty 4555 decoder. I think if the LF13331 had been the problem, the LED on the panel would have been showing the saw mode, but it wasn't.

Upon re-assembly I tried everything out and came to notice that the lower order harmonics were coming out as just a sin wave. When I tweaked the trimmer I got a little action, but only about 25% of the total that is normal for the wave folder section. It turns out, the 2N3958 dual FET transistor that is supposed to be there was substituted with 2 2N4339 FETs. These FETs don't have as much gain I guess, because they weren't doing the job. Rather than tweak resistor values, I put in the real thing and it worked perfectly!

Now it's working properly. I even added some rubber caps to the switches(not in the picture). I don't mind the black knobs, in fact I prefer them to the unskirted ones I got, but I HATE the blue LEDs.